As mentioned, the water we use today has been around for hundreds of millions of years, and the amount available hasn't changed very much. Water moves around the world, changes forms, is taken in by plants and animals, but never really disappears. It "travels" in a large, continuous cycle called the Hydrologic Cycle.
The Distribution of Water on Earth
More than 70% of the Earth is covered by water. Nearly all of that water is contained in four oceans: the Pacific, the Atlantic, the Indian and the Arctic. The rest can be found in seas, bays, gulfs, lakes, rivers, polar ice-caps, ground water and in the air or
atmosphere. All of the various water systems can be classified into two types of water: salt water - oceans, seas, and fresh water - rivers, lakes, glaciers, polar ice-caps, rain, ground water
The States of Water
Water exists on Earth in each of its three states:
solid - polar ice-caps, glaciers, icebergs
liquid - oceans, seas, lakes, rivers
gas - air/atmosphere
All water on Earth is constantly changing from one state to another. The addition or removal of heat will lead to changes of state:
The circulation of water through its states is the Water Cycle
a) Evaporation - water from lakes, rivers and oceans changes to water vapour by the
action of the sun.
b) Transpiration - water vapour is released from plants by the action of the sun.
c) Condensation - water vapour changes to droplets of liquid as it cools, leading to the
formation of clouds.
d) Precipitation - water from clouds is released back into lakes, rivers and oceans, as
rain, snow or hail, or onto land where it seeps through the soil and travels as ground
water back to the ocean.
e) Melting - ice and snow change to liquid water by the addition of heat, and flow
into lakes, rivers and oceans.
Salt water and Fresh water
Approximately 97% of the Earth's water is salt water and found in seas and oceans. Only 3% of our global water supply is fresh. Most of this fresh water is not being used. It is locked up in glaciers and polar ice-caps. The rest is located in rivers, lakes, (water vapour in the air) and in the form of groundwater.
The average ocean salinity is 35 ppt. ( parts per thousand ) This number does vary, usually between 32 and 37 ppt. Rainfall, evaporation, river runoff and ice formation cause the different ppt's.. For example, the Black Sea is so diluted by river runoff, its average salinity is only 16 ppt.
Freshwater salinity is usually less than 0.5 ppt. Water between 0.5 ppt and 17 ppt is called brackish. Estuaries (where fresh river water meets salty ocean water) are frequent examples of brackish waters.
If over 70% of the earth is covered in water, then why all the concern? We have lots.
Why can't we just use the seas and oceans? Isn't there a way to get rid of the salt?
Perhaps we can melt all the glaciers.
What's all this talk about acid rain? Is that polluting the rivers and lakes? Is the acid rain seeping into the ground and polluting the ground water?
What is ground water. Is it just the water the makes the soil feel wet?
How and why does a geyser shoot up water?
Some of my friends buy and drink bottled water. Is it really better than my tap water? Is all bottled water the same? Is spring water the same thing?
Think about some of these questions? What are your ideas and opinions?
People upset the natural water cycle. That is because we:
1) create pollution,
2) remove the natural vegetation such as plants and trees,
3) pave over the natural environment and build houses etc.,
4) put in irrigation systems, build dams, change natural drainage systems, build wells,
5) ? Can you think of any ways people disturb the hydrological cycle?
We have SO much water in our world yet many places are crying for water.